Diurnal Profiles of Gastrointestinal Regulatory Peptides

Abstract
The gastrointestinal tract is the greatest endocrine organ in the body, containing a large number of biologically active peptides (1.2). Several of these peptides function both as circulating hormones and as neurotransmitter substances; some act solely as circulating hormones: some are localized to nerve terminals and probably function as neurotransmitter substances only, and others are most likely biologically active locally in the tissues, having mainly a paracrine effect.